Plant materials are routinely assessed to determine their quality, viability or fitness for a particular purpose: Seeds, seedlings and propagating materials such as tubes, cuttings, bulbs and the like are assessed for their viability and growth potential prior to planting. Other agricultural products, such as fruits, grains, flowers and vegetables are also assessed for viability, freshness, consumability and storage life.
Assessment of viability or quality is particularly important when plant materials have been stored for extended periods or have been transported over long distances. Plant materials are subject to a wide range of stresses during storage and transport which can have serious affects on their subsequent viability and quality. Stresses to which plant materials may be subjected during storage and transport include cold temperatures, freezing, excessive heat, water deficit, physical stresses and chemical stresses from pollution. Although such stresses can seriously impact on the viability of plant materials and on their future potential for growth and storage, the affects may not be immediately apparent even from a close visual inspection of the materials.
The need for a method to assess viability of plant materials is particularly acute in the forestry industry where large scale seedling planting is an important aspect of reforestation and protection of renewable wood resources. Billions of tree seedlings and batches of seed are shipped annually for planting, frequently in remote areas. The recipients of the seeds and seedlings are frequently unaware of the stresses and damage to which the seeds and seedlings may have been subject, unless the damage is so severe as to be apparent from a visual inspection. Considerable expense and effort are involved in the planting and nurturing of young tree seedlings, which are frequently planted in large numbers over vast areas. Significant losses are incurred where stressed seedlings are planted which have poor viability and reduced growth potential, resulting in non-reforestation or low yield.
Typically the areas where replanting is to take place are in remote locations. Thus tree seedlings or plants to be planted must be transported to these locations from centrally located nurseries. Transportation may stress the trees, making them weaker and less viable or less suitable for planting. Because of the manual labour and expense involved in planting trees it is desirable to have as high a survival of planted trees as possible, namely that the tree seedlings be as fit as possible for replanting. In this manner seedlings have a maximum chance of surviving and growing vigorously. This helps in avoiding costly replanting.
One of the problems with tree seedlings is that it can be very difficult to evaluate plant viability in the field. To date visual inspections have been the main method of evaluation of plants immediately prior to planting. Such assessments though may not detect that the plant has been recently stressed in a way and to an extent which negatively impacts the seedlings' viability. In addition, such stress may not visibly manifest itself until many days or even weeks later.
The viability and quality of plant materials may be assessed by various laboratory analytical procedures to assess the affects of damage and stress. For example electrolyte leakage and chlorophyll fluorescence may be measured in the laboratory. The gases evolved by plants have been subjected to gas chromatographic analysis in the laboratory. Some trees have been shown to produce elevated levels of ethylene, ethene, acetaldehyde and ethanol in response to certain stresses (Kimmerer, T. W. and Kozlowski T. T. 1982. Plant Physiol. 69: 840-847).
Deterioration of seed has been shown to be accompanied by production of volatile gases, such as ethanol and acetaldehyde (Woodstock and Taylorson (1981) Plant Physiol. 67: 424-428; Crawford (1977) New Phytologist 79: 511-517). The viability of seeds is important as fields shown with poor quality seed may experience reduced yields at harvest due to lack of, or slow, germination. Furthermore, in agriculture, seeds of some plant species are consumed directly (e.g. rice) or are processed into food products (e.g. wheat and other grains for floor production).
Increased levels of acetaldehyde and ethanol in imbibing bean seeds subjected to accelerated aging treatments have been reported by Lowell, W. W. and Taylorson, R. B. (Plant Physiol. 1981, 67:424). Ethane levels in the head space gas from incubation tubes containing leaves or seedlings of loblolly pine have been measured by gas chromatography and correlated with electrolyte leakage as a possible method for assessing freeze damage (Johnson, J. D. and Gagnon, K. G. 1988, New Forests 2:65-72).
A wide variety of volatile emissions are also given off by several fruit species. (Nursten and Williams (1967) Chem. Indust. 486-497; and Mawele, S. M. et al, 1992, J. Food Sci. 57:1173). Fruits and vegetables produce many volatile compounds, for example, 37 compounds were identified by Nursten and Williams in pineapple (Ananas sativus L.). Some of these gases, such as ethanol, acetaldehyde and ethane, are related to stress and damage (Bressan et al. (1979) Plant Physiology 63: 924-930). However, the physiological significance of most of these compounds have not been elucidated.
There is no rapid and inexpensive method for field evaluation of volatile emissions produced by seedlings.
There is a need for a portable, inexpensive, non-destructive and reliable method for assessing viability and quality of plant materials outside the laboratory setting. In particular, foresters, nursery managers and farmers require a method for rapidly assessing damaged plants when damage occurs close to the time of shipping or planting. In forestry, screening for seedling viability is currently available at centralized facilities and takes at least 48 to 72 hours to obtain initial results and thus cannot provide the fast response required for on-site operational decisions.